When place a work 3 on the Propraetor action space and choice play card to increase my influence.
I only increase influence Area 3 ? Is that's correct?
Yes. The number of the worker you use determines the area that you gain resources from and where you can increase your influence.
Rule important say:
If you already have a marker on the area being used, then this marker must be moved.
What's the mean?
I assume this is just inferring that you can only have one influence marker on each area influence track.